Chicago — Marnell Brown's phone rang an hour before midnight. A 17-year-old youth had been shot and killed not far from his office in West Garfield Park, one of Chicago's poorest and most violent neighborhoods. Some of the teen's friends were gathered a few blocks away, the caller said. They wanted to strike back.

Mr. Brown went to stop them. "So far we have a casualty, one lost," he remembers telling them. "You're willing to go to war? You'll lose more. And what about civilians? Are you prepared for that – friends and neighbors, brothers and sisters? You may hit your target. But when they come to retaliate, they ... might hit your mother or sister."

As cities struggle to curb gun violence, more of them are turning to people like Marnell Brown. A former gang member who spent much of his adulthood in prison, Brown works on some of Chicago's most ruthless streets, steering young men away from violence and toward jobs, schools, drug treatment, and more stable lives. At times he and his colleagues try to restrain agitated, angry, and often drug-addled youths from the kind of retaliatory violence that characterizes much gang conflict.

Gun violence, especially involving gangs, continues to baffle many cities. Gang-related killings soared from the 1970s until the mid-1990s and has remained high ever since. According to the federal Centers for Disease Control and Prevention, homicide is the leading cause of death among African-Americans ages 10 to 24.

Tougher policing has been one answer. In Chicago, the police recently announced new measures to cope with a spring upsurge in shootings, such as equipping officers with powerful rifles and sending them in SWAT-like gear into troublesome neighborhoods.

Increasingly, however, cities are looking for strategies that go beyond law enforcement and strike at the causes of gun violence. These strategies typically involve helping communities resist violence from within and weaning young men from reliance on gangs. In Los Angeles, a city that has been struggling with gang warfare for decades, officials have been trying to broaden their approach after a 2006 study found numerous shortcomings in previous antigang efforts, including an overreliance on policing.

"You've got to stop the flow of young people into gangs in the first place," says Jeff Carr, deputy mayor for gang reduction and youth development in L.A. "If you arrest one guy and there are three waiting to take his place, you're not going to solve the problem."

One effort that has attracted attention is the Chicago Project for Violence Prevention and its antiviolence campaign, CeaseFire. Based at the University of Illinois's Department of Public Health, CeaseFire treats gun violence as a public health problem and fights it by trying to change individual behavior and community norms. The police "catch people after they cross the line," says Gary Slutkin, an epidemiologist who started CeaseFire eight years ago. "Our job is to keep them on this side of the line."

Working mainly in poor black and Hispanic neighborhoods in Chicago and its suburbs, CeaseFire promotes nonviolence with such things as leaflets, bumper stickers, and signs announcing how many days have passed since the last shooting. It works with local ministers and other community leaders to organize neighborhood marches, rallies, and vigils after shootings.

Meanwhile, outreach workers recruit and work with a small number of youths deemed most at risk of shooting or getting shot. "Violence interrupters," and sometimes the outreach workers themselves, intervene when violence breaks out, often seeking out a victim's friends and relatives to discourage retaliation.

CeaseFire typically hires ex-gang members who have done prison time, found religion, and joined the campaign to earn a living and give back to their old neighborhoods. It trains them how to resolve conflicts, connect with clients, and stay out of trouble. But their success depends heavily on their insider's knowledge of gang culture and on the credibility they enjoy on the streets.

A slender, personable man of 49, Brown grew up on Chicago's West Side, where he mastered "short cons" like the pigeon drop and three-card monte. He also robbed, beginning with the holdup of a mail carrier when he was 16. He spent almost two decades in prison for a variety of crimes, including armed robbery and aggravated assault. Three years ago, out of prison and working as a drug counselor, he began working for CeaseFire.

Most of Brown's clients are members of local "crews," the small, loosely organized neighborhood gangs that rule Chicago's poor African-American communities. Many have already done jail time. "I have an overwhelming sense of compassion to see guys not go through what I went though," he says.

Brown says violence usually erupts not over drugs, money, or turf but over "just nonsense" – some real or imagined slight, a quarrel over a woman, an old grudge. The victims are often bystanders, including children. Brown tries to get young men to think before they act, often by asking about their families.

"When you talk to people about family, you get in touch with their compassion," he says, and "once the spirit of empathy is aroused, it's hard to miss."

Antiviolence efforts have long been hampered by uncertainty over what really works, experts say. "We have almost no program or policy research on what is effective in addressing the gang problem," says Irving Spergel, a retired University of Chicago professor and an authority on gangs.

A new study out of Northwestern University, however, concludes that CeaseFire has reduced gun violence in many Chicago neighborhoods. The study, released earlier this month, says outreach workers have been especially effective in befriending young gang members.

"After their parents, the outreach workers were named as the most important people in their lives," says Wesley Skogan, the lead researcher.

It's not an easy approach. Many of the young men involved with gangs have acquired little aptitude for work or school. In Los Angeles and elsewhere, outreach workers themselves have sometimes lapsed back into illegal activities.

And public support has been unreliable. Last year, CeaseFire had to close most of its neighborhood offices in Chicago and elsewhere in Illinois after the state legislature cut many social service programs out of the state budget, forcing the campaign to rely largely on private donations.

But CeaseFire remains popular on the streets. Marshall Hatch, pastor of Chicago's New Mount Pilgrim Missionary Baptist Church and an old ally of the campaign, says local people feel "a sense of pride … when ex-offenders are working on the right side."

And the campaign is expanding beyond Illinois. CeaseFire is in the second year of a four-year effort, financed by the US Justice Department, to replicate its work in other cities, including Newark, N.J.; Kansas City, Mo.; and Baltimore. "If we can use respected people from these communities to change behaviors related to guns, we can save lives as much as a new police strategy," says Joshua Sharfstein, commissioner of Baltimore's health department.

Marnell Brown is still at work. On a recent afternoon, 19-year-old Tyrone Lockett came looking for him at the CeaseFire office on West Chicago Avenue. Mr. Lockett said he had been trying to find work for a year and a half and that Brown was helping him "get his papers in order." Under Brown's guidance he had already found a place in a residential drug treatment program. "I basically just stick to myself, focus on my own business," he said.

For Brown and many others, keeping young men focused on their business and not on guns is one of the keys to making neighborhoods safe. But it takes skill, instinct, and perseverance.

"It's more than talking to you to try to stop you," Brown said before driving off in search of a high school transcript. "I need to keep you stopped. It's easy to stop someone from committing violence. It's hard to keep them from starting again."